This invention relates generally to distributed radio systems and, more particularly, to an indoor system with multiple transceivers for simulcasting and selective processing of received signals.
Wireless (e.g., cellular) operators are faced with the continuing challenge of expanding the coverage areas of their systems, while at the same time adhering to cost, power, and frequency limitations. One area for expanding coverage is in indoor environments. However, it is also desired that such systems not interfere with the existing macrocellular environment, even if operating in the same frequency band for a given cellular standard.
The present invention is directed to providing a system and method that can be operated at low power levels and which cause minimal interference with the existing macrocellular environment. More specifically, the present invention is directed to a system and method in which multiple transceivers are utilized in an indoor system and which are able to each effectively transmit signals to a mobile unit at low power levels by utilizing a simulcasting technique.
A distributed radio system with multiple transceivers for communicating with mobile units via simulcasting and selective processing of received signals is disclosed. In accordance with one aspect of the invention, the system transmits at very low power levels. Due to the use of the low power levels, the system does not tend to interfere with the existing macrocellular environment.
In accordance with another aspect of the invention, the system utilizes relatively few frequency channels. The system is able to provide large geographical coverage with relatively few channels, while transmitting at low power levels, by simulcasting identical radio frequency signals from several different radio transmitters distributed within a building or similar area using the limited number of frequency channels. This allows the system to cover a relatively large indoor installation. Furthermore, as noted above, the system interferes very little with the existing macrocellular environment, even though the same frequency band for a given cellular standard is being used, given that the system is able to operate at very low power levels by utilizing the simulcast technique.
In accordance with another aspect of the invention, in one embodiment the distributed radio system includes a plurality of processing elements and radio frequency transmitter elements interconnected by an Ethernet network (e.g., the IEEE 802). The distributed radio system simulcasts a common modulating signal on a common radio frequency carrier, using at least two radio frequency transmitter elements. A set of radio frequency transmitter elements, which simulcast a common modulated radio frequency signal, are designated as a radio frequency simulcast set. In operation, the elements of a radio frequency simulcast set are configured to receive Ethernet sampled signal packets and transmit the information contained in the packets by modulating their radio frequency carrier.
In accordance with another aspect of the invention, in one embodiment the simulcast method designates a number of particular radio frequency transmitter elements to be elements of a radio frequency simulcast set. A first set of Ethernet packets is transmitted to be received by the elements of the radio frequency simulcast set. The first set of Ethernet packets is used to program the elements of the radio frequency simulcast set with a multicast address. The elements of the radio frequency simulcast set are thereafter responsive to Ethernet packets containing the multicast address as the designation media access control (MAC) address. A second set of Ethernet packets is transmitted to be received by the elements of the radio frequency simulcast set. The second set of Ethernet packets is used to program the elements of the radio frequency simulcast set to operate using a particular common radio frequency carrier. In addition, Ethernet sampled signal packets are periodically transmitted. The Ethernet sampled signal packets contain a designation address, which is equivalent to the programmed multicast address. The elements of the radio frequency simulcast set are configured to receive the Ethernet sampled signal packets. The elements of the radio frequency simulcast set modulate the common radio frequency carrier in accordance with sampled data included in the Ethernet sampled signal packets.
In accordance with another aspect of the invention, the processing elements of the system include at least one central processing unit and a plurality of airlink processing units. The central processing unit is responsible for interfacing the system to external environments, such as a macrocellular environment, or to a public switched telephone network, as well as for network management of the overall system. In one embodiment, the central processing unit may be a network chassis unit. The central processing unit is coupled to several airlink processing units through an Ethernet network. Each airlink processing unit is coupled to several radio transceivers, also through the Ethernet network. In one embodiment, the airlink processing units may be airlink chassis units.
In accordance with another aspect of the invention, the data link layer may be centralized. For the transmission of data to the mobile units, it is important that the data, which is simulcast by several transmitters, be identical. If each simulcast radio frequency transmitter element does not transmit identical signals, co-channel interference can result, thus degrading the signal received by the mobile unit. To ensure that the transmitted data is identical, the layer 2 (data link layer) is centralized at the central processing unit. Layer 2 Ethernet packets are sent by the central processing unit to several airlink processing units, which further process the layer 2 information into waveforms, which are finally transmitted by each simulcast radio transmitter.
In accordance with another aspect of the invention, transmissions may be implemented by a two-level multicast technique. In an embodiment that utilizes this technique, the transmit data is sent by the central processing unit to multiple airlink processing units in layer 2 format as multicast Ethernet packets. Then, the transmit data is sent by each airlink processing unit to associated multiple radio transceivers in layer 1 format (sample waveforms), also as multicast Ethernet packets.
In accordance with another aspect of the invention, when signals transmitted from a mobile unit are detected by multiple radio receivers, the system is able to select a desired radio receiver signal for processing. The multiple radio receivers simultaneously provide detected data into the system. The detected data includes identical information, but at different quality levels. The system is able to select a desired signal for processing.
In accordance with another aspect of the invention, when selecting a received signal for processing, a distributed processing technique is utilized that performs a process of gradual selection. A given mobile device may be located in relatively close proximity to several possible wireless receivers. It is desired to select and process only the signal that is provided by the receiver having the strongest signal from the mobile unit. A selection decision is made by a process that has visibility to all of the relevant receivers in the system.
In accordance with another aspect of the invention, in a preferred embodiment, the system is interconnected via Ethernet links having limited bandwidth. This configuration would generally make it impractical to forward data from every receiver in the selective decision process because such would tend to rapidly exceed the network capacity. The solution provided by the present invention is to utilize a distributed processing technique that results in gradual selection. More specifically, the selection process may be performed at several different levels, including at the central processing unit and at the airlink processing units. The final selection process is located in the central processing unit, which is at the top of the system hierarchy. However, each airlink processing unit will choose the received signal from only one of the radio transceivers and will forward only one signal to the central processing unit for a final decision. In this way, the simulcast receiver processing is distributed through the system and the selection of the best-received signal is gradually selected as received signals flow up the hierarchy.
In accordance with another aspect of the invention, the selection process may be more or less distributed, depending on the available data rates of the system. For example, a system utilizing the Global System for Mobile Communications (GSM) technology, which has a relatively high data rate, may allow the central processing unit to have sufficient time to receive signals from several radio processing transceivers in a given airlink processing unit as part of the selection process. An additional technique for screening signals may involve requiring a minimum signal strength, which is detected in the radio transceivers themselves, in order to be forwarded upstream to the corresponding airlink processing unit.
In accordance with another aspect of the invention, the distributed processing utilized by the selection process may be synchronized. In one embodiment, it is desirable that the signals received from each radio transceiver correspond to substantially the same moment in time in order to correctly evaluate the differences between the received signals. It is desirable that the central processing unit be synchronized with the airlink processing units and the radio transceivers so that the central processing unit knows when to expect the arrival of received signals from the airlink processing units and when a decision in the selection process should be made. It is preferable that the synchronization be performed at least at the end points of the system, i.e., the central processing unit and the radio transceivers. However, the system delay can be further minimized if the processes in the airlink processing units are also synchronized.
In accordance with another aspect of the invention, a selection time window may be set for making the selection process for the upstream-received data traffic. Thus, selection decision is made by the selection process in the central processing unit within a certain time window, regardless of whether all expected received signal data has arrived at the central processing unit. In general, as part of this method, the central processing unit has a time base that is synchronized with the airlink processing units and the radio transceivers.
In accordance with another aspect of the invention, the data traffic may be bundled. Inbound traffic received from the radio transceivers is bundled in order to not overwhelm the central processing unit, which services the task for transmitting Ethernet packets. For multiple radio frequency channels, multiple Ethernet packets, each containing a selected received signal, can be periodically sent from each airlink processing unit to the central processing unit. Rather than sending individual Ethernet packets for each radio frequency channel, the data is bundled into one or more Ethernet packets, thus reducing the overhead required to service the transmission of multiple Ethernet packets. Outbound traffic that is transmitted by the radio transceivers may also be bundled, such that information from multiple radio frequency channels is grouped into a single Ethernet packet.
In accordance with another aspect of the invention, the system may perform selective management of Ethernet switches. In certain areas of the system, the data transmission traffic may be very high, such as at the Ethernet link between the radio transceivers and the airlink processing units, since much of this traffic represents the raw samples of the waveforms. Many of these Ethernet packets are also multicast packets, which would ordinarily be flooded throughout the network. However, in some embodiments this may be undesirable because other network connections in the system, such as between the central processing unit and the airlink processing units, may be burdened or overwhelmed by the traffic. In large systems, it is desirable to configure the Ethernet switches to filter, or reject, the transmission of designated packets on designated ports.
It will be appreciated that the disclosed system and method are advantageous in that they can be used to provide large geographical coverage (e.g., in an indoor system) with relatively few frequency channels and with relatively low power transmission requirements, and that they thus minimize the interference with the existing macrocellular environment even while operating in the same frequency band for a given cellular standard.